H2—O2(air) fuel cells are well known in the art and have been proposed as a power source for many applications. There are several types of H2—O2 fuel cells including acid-type, alkaline-type, molten-carbonate-type, and solid-oxide-type. So called PEM (proton exchange membrane) fuel cells [a.k.a. SPE (solid polymer electrolyte) fuel cells] are of the acid-type, potentially have high power and low weight, and accordingly are desirable for mobile applications (e.g., electric vehicles). PEM fuel cells are well known in the art, and include a “membrane electrode assembly” (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack.
In PEM fuel cells hydrogen is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can either be in a pure form (i.e., O2), or air (i.e., O2 admixed with N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprise finely divided catalytic particles (often supported on carbon particles) admixed with proton conductive resin.
During the conversion of the anode reactant and cathode reactant to electrical energy, the fuel cell, regardless of the type, produces anode and cathode effluents that are exhausted from the fuel cell. The anode effluent typically contains unused hydrogen that represents an unused source of energy. The cathode effluent typically contains excess oxygen or air that was not consumed during the electricity production in the fuel cell. The amounts of hydrogen and oxygen remaining in the anode and cathode effluents is dependent upon a number of factors and will vary. For example, the efficiency of the fuel cell can impact the amount of hydrogen and oxygen that are exhausted in the respective anode and cathode effluents. Additionally, the stoichiometry of the fuel cell stack (i.e., the amounts of hydrogen and oxygen that are included in the respective anode and cathode reactants) will also effect the amount of remaining hydrogen and oxygen in the respective anode and cathode effluents.
The hydrogen in the anode effluent represents a source of energy that can be converted into a more usable form. Typical fuel cell systems employ a tail gas combustor to convert the hydrogen in the anode effluent into heat that can be used in other parts of the fuel cell system. However, the conversion of the excess hydrogen to heat may not be the most efficient use of the energy contained in the anode effluent. The tail gas combustor produces emissions that may require additional processing before the emissions can be vented to the environment. The heat generated by the combustor, may only be needed during certain aspects of operating the fuel cell system, such as at start up, and thereafter become a source of lost energy in the form of heat that must be dissipated from the fuel cell system. The tail gas combustor operates at high temperature. The use of a tail gas combustor also requires additional controls and/or control schemes that differ from the controls and/or control schemes to operate the fuel cells. All of the above considerations increase the complexity of a fuel cell system incorporating a tail gas combustor. Therefore, it would be desirable to convert the energy in the anode effluent into a more useful form without the necessity of creating excess heat, emissions and/or requiring additional and/or different controls/control schemes.